Background from Alan Melia.....
“Hi Roger you can calibrate your loop first using DCF39 on 138.83. This has a field strength in our area of 1mV/m. Then you could work back your own ERP by comparison...... 1W ERP produces 300mV/m at 1km, 100mV/m at 10km (from memory
....I think that is correct....10dB per decade after 1km). Doing it that way removes as lot of the variables, and its a good thumbnail calculation.”
So, this afternoon I measured the strength of DCF39 as
-22.8dB using the same loop and receiver and Spectran settings, so -22.8dB is equivalent to 1mV/m field strength based on Alan’s assumptions.
G3XBM’s signals at 14.9km measured -42.5dB which is approximately
100uV/m. Assuming 100mV/m at 10km equals 1W ERP, then 64mV/m would equal 1W ERP at 14.9kms (approximately 4dB weaker signal from the CCIR curves).
The ERP is lower by the ratio of 100uV/m to 64mV/m. So, my ERP in the best direction is 56dB down on 1W, which is
2.5uW ERP.
If this calculation is correct then I’m surprised that such a low power (2W from PA) into such a strange antenna structure (20m separated ground rods connected
by wire just 1.5m above ground for most of the run) can produce so much ERP.
Please can you check these figures Alan (or others) and see if you agree?
73s
Roger G3XBM
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